The Clarets’ run of 25 Championship games without defeat — stretching back to late 2015 — came to an end at Vicarage Road.
The visitors once again bossed possession, owning 65% of the ball, but conceded the only goal of the game with what was Watford’s only shot on target.
Tom Cleverley passed the ball into the net right on the stroke of half-time after being picked out inside the penalty area by Ken Sema.
Josh Brownhill almost salvaged a point for the away side with the last kick off the game, but the midfielder’s volley from substitute Vitinho’s cross struck the crossbar.
Here are Chris Boden’s player ratings.
5. Charlie Taylor 6
Competed well against Watford's physical forwardline. Photo: Richard Heathcote
6. Josh Brownhill 6
Quiet first half, but had a raft of decent openings in the second, foiled by the keeper and woodwork. Photo: Ashley Allen
7. Manuel Benson 5.5
Clattered three times early on by Kamara, who wasn't punished, and was reluctant to go on the outside after that. Photo: Andy Ford/Burnley FC
8. Josh Cullen 7
Another who put his foot in, but passing not at the levels of Huddersfield. Photo: CameraSport - Rob Newell